You may also see that a website called JustFly is showing for a few dollars cheaper. Personally, I'd stick to booking it through Priceline, as they're known for being one of the best for honoring fares that are actually mistake.

I'm not 100% sure this one is a mistake, but the same flight is showing up as $499 on Air Transat's website.

On Priceline it should show up as $133 USD, which should bill as approximately $166 CAD on a Canadian credit card based on today's exchange rate, and factoring in the typical 3% currency exchange fee.